(KRV, RV, Rat Virus, RPV, parvovirus r-1)

Definition

There are three serogroups of parvoviruses that affect rats: RPV (formerly rat orphan parvovirus), RV, and H-1 (Toolan’s). Kilham rat virus is one strain representing the RV serogroup.

The Parvoviridae are single-stranded DNA (ssDNA) viruses. Having no cell structure of its own it infects the body’s cells to replicate. Parvoviruses preferentially replicate when cells are in a mitotic stage of cell division such as is seen during fetal development.

Clinical Signs

RV infection is usually subclinical. Within an infected colony there may never be any sign of illness other than an occasional small litter, or a temporary lack of litters produced.

During the active stage of the infection clinical signs may include:

• Small litters
• Infertility
• Runting
• Stillborn babies
• Litter resorption

On rare occasions RV can become clinically evident in unnaturally infected rats. If this occurs the rat may experience failure to thrive, ataxia (uncoordinated muscle movements), cerebellar hypoplasia (congenital damage to the part of the brain that controls coordination of movement), or jaundice (yellowing of the skin & mucus membranes due to the liver’s inability to process bile). These signs have been seen in rats experimentally exposed to virulent strains of RV during the late stage of pregnancy or the first week of life. Scrotal cyanosis has also been seen in rats experimentally infected with RV.

Etiology

Rats are a natural host of Kilham rat virus. Members of the RV serogroup is more likely to be associated with clinical disease, but infections may also be subclinical.

The modes of transmission are horizontal through exposure to infected urine, feces, nasal secretions, milk, fomites (inanimate objects or materials which can convey disease-producing agents), and through the placental barrier. Although transmission is not typically by aerosol, the viruses it can become airborne on particles of dust during the disturbance of the rats’ bedding or litter.
Vertical transmission is possible; however, it appears to require large doses of the virus, which helps to explain why prenatal infection is uncommon during natural outbreaks in breeding colonies (Kilham and Margolis, 1966, Jacoby et al 1988, Gaertner et al 1996, Kajiwara et al 1996).

Parvoviruses target replicating tissue and can cause cell and tissue destruction. This explains why RV, while in its active stage, will interfere with the formation of offspring while they are developing in utero or perinatally. Such interference may result in smaller litters, stillborns, or the resorption of litters.

Antibodies will form within 7–10 days after exposure. It has been concluded through research that the anti-viral antibodies do not clear infection (Ball-Goodrich et al., 2002).

If an exposed female has already built-up antibodies, they will be passed on to her offspring in colostrum during suckling. After the transference of the antibodies these young rats will test positive but will not be infected. These babies may then become infected 2–7 months after the effects of maternal antibodies have worn off. At this point they would have less evidence of clinical disease.

If the mother has been infected and has not had time to build up antibodies, then the developing babies can be affected in utero. Infection early in the pregnancy may lead to partial or total resorption of the fetuses. If a dam is infected in the last trimester (with no antibodies present) she may then transfer the virus either in utero or through milk, after they are born, to the surviving offspring. Although, this appears to require very high doses of virus to achieve-references as above.

Infection occurring from the third trimester in utero through the first week of life can result in rats with a more persistent infection. Early infection such as this can take up to six months to clear. This only appears to occur if the fetuses belonging to a non-immune dam are infected within a very specific, narrow window of time, making it unlikely to be a concern in the domestic situation (Jacoby et al, 1990).

The persistence of the virus depends largely on the age and/or immunocompetence of the rat when it is exposed. The younger the rat is at initial infection the longer the disease will persist. Young rats are immunologically naïve and therefore stay infectious much longer than adults. As above, this appears to be less of an issue than previously thought.

Once the rats get past the active infection, typically 60–90 days, the virus does remain in certain tissues including the spleen, lymph nodes, and walls of blood vessels. These areas of infection are not affiliated with the transmission routes (such as the urinary tract, mammary glands, and digestive system) and therefore the virus will no longer be transmitted via urine, milk, or feces. Note that once a rat is exposed it will continue to test positive for RV on serology (antibody) tests even after the active infection as cleared.

Research has shown that if a normally post-infectious rat is stressed either physically or psychologically there is a possibility that the RV can be reactivated and transmitted to other rats. The thinking now is that this would be a very rare occurrence in the domestic situation with immunocompetent rats (A. Wharton, BVSc, MRCVS, CertGP, personal communication with multiple laboratories, 2024).

Other factors that contribute to infection and transmission issues are the rat’s natural resistance to disease, whether the rat is immunocompromised (e.g., athymic), and the particular strain of RV you are dealing with. In a rat with immune system disorders, it would be possible for RV to persistently infect. Particularly virulent strains may persist past the normal predicted times.

Diagnostics

A colony showing a decrease in pregnancies and litter size may be infected with RV. If you feel serological testing is needed you, or your vet, can contact a testing lab to get specific instructions on how to prepare a test sample. It is important to remember that other syndromes can lead to litter loss and reproductive issues such as uterine mycoplasmal infections and/or nutritional issues.

RV antibodies can show up on a standard serum test as soon as seven days post-exposure, but it is recommended to wait 14–21 days after possible exposure for more accurate testing. The rat being tested should be at least two months of age. Remember that young adult antibody-positive rat may have maternal antibody and not antibody developed as a result of active Infection.

If your vet does not have the equipment to spin the blood, then the lab can explain alternate methods. Usually .5mL of whole blood is required to make between 0.1—0.25mL of serum (minimum for one test is 0.1 mL). Pooling blood will decrease the accuracy of your results. Do not add anticoagulants. If the sample is diluted with saline, please note the ratio (usually 4 parts saline—1 part whole blood). Samples can be iced and shipped overnight.

The ELISA is used for initial screening with RV as part of the basic series. If the RV test is positive, other more specific tests discriminating among H–1 virus, RV and RPV infections can be run. You can specifically request RV (KRV) testing.

ELISA (enzyme-linked immunosorbent assay)—primary screen
IFA (indirect fluorescent antibody assay)—confirmatory test

Antibody tests simply show that a rat has been exposed to RV at some point during its life. This does not necessarily imply and active infection – therefore may be an incidental finding in a colony.
Polymerase Chain Reaction (PCR) testing detects particles of virus DNA, which will only be present during an active infection, and is therefore a much more relevant test to use when attempting to determine if RV is responsible for clinical signs being observed. This test, also, has the advantage that it can be done on samples other than blood – if a rat is actively shedding virus, particles will also be present in feces, urine, and body tissues for example body tissues from aborted fetuses.

Recommended Testing Facilities:

• IDEXX RADIL, University of Missouri, Columbia, MO:
  https://www.idexxbioanalytics.com/radil-advantage-program-profiles#rat
• Charles River Laboratories, Wilmington, MA: (800) 338-9680
• Surrey Diagnostics in the UK are able to do both antibody testing and PCR

Treatment

There is no medical treatment for RV Infection.

Impact on Pet Owners/Breeders

RV is not known to be a health concern in pet rat colonies. Even so, it is advisable to stay cognizant of the potential and keep an eye out for subtle signs of infection.

In the breeding environment, it may affect the outcomes of planned breeding. It is not typically a health issue and can be active in a colony with little or no effect.

In a colony, the RV infection can be kept active by the addition of naïve rats, either in the form of new litters (see previous notes about the likelihood of vertical transmission) or by bringing in rats from outside sources.

Once rats have shed the virus, they are not typically contagious to others, but they will continue to test positive for RV (antibody) and there is no conclusive proof that it can no longer be transmitted. It is possible (though very rare in an immunocompetent individual in a domestic setting) for the RV Infection to become reactivated in a rat, thus infecting new additions. It is also possible for new rats to become infected via fomites.

A positive ELISA does not necessarily mean the rat is infectious to others. A positive result can simply mean that the rat has been infected at some point in the past and is no longer shedding the virus. DNA testing performed on excreta can indicate whether or not virus is still being shed. More Invasive (terminal) procedures are required to show whether the rat Is Infected at sites not expected to be associated with transmission. DNA testing may not be a practical option for the pet owner or hobby breeder because it is very expensive and may require euthanasia of the rat. Certainly, in the UK, this is not prohibitively expensive, and does not require euthanasia in order to perform.

Attempting to Clear a Colony of RV

Some breeders and fanciers may choose to attempt to clear their colonies of RV. It may be possible to do so although there are factors that make such an attempt difficult. The effort would mean that the fancier would have to consider not only the rats, but also the environmental aspects. Even with the strictest procedures there are no guarantees.

Parvoviruses are extremely stable in the environment and can remain viable for weeks or months. It is this characteristic that facilitated the worldwide spread of canine parvovirus in the early 1980s. Removing the virus from an environment, especially in a non-lab setting, would be monumental. The first step would be to thoroughly scrub to remove all debris on cages, accessories, walls, and floors with soapy water then sterilize with bleach solution. No surface, crack, corner, or wire could be ignored if the clearance is to be as complete as possible. Carpet, furniture, and drapes may present a problem during an attempt to disinfect. Dishes, bowls, and water bottles would also need this treatment. Porous items such as wicker nests and wood accessories could potentially harbor the virus and elimination may need to be considered. Food and litter, even though unused may also have been contaminated during normal handling and may also need to be eliminated. This would have to be completed after the quarantine and well before any rats were brought in.

Attempting to clear a colony of KRV would have to start by following a quarantine of 60–90 days for juvenile and adult rats and up to six months for rats suspected of in utero infection or within one week of birth (likelihood of vertical transmission low as per previous notes).

Moving the rats exposed as infants (or in utero) to another location would decrease quarantine time. There would have to be a complete moratorium on breeding and acquisition of new rats.

Cleaning Agents for Parvoviruses

• Household Bleach (4 oz bleach—1 gal water) Do not store mixed solution
• PARVO Disinfectant Spray by Simple Solution

  After a full quarantine and a thorough cleaning, the success of the campaign can be tested by bringing several new rats into the colony (making sure they are negative for antibody before introduction), distributing them throughout the subgroups and then ELISA testing them two – three weeks [prefer 4 weeks If low-level contamination Is still present] later. Negative testing will not be conclusive proof that the colony is 100% free of active RV but may serve as an indicator.
  
  Keep in mind that it is also possible for the previously infected rats, through reactivation, to transfer the disease to the new rats at any given time causing active infection (again theoretically possible, but very unlikely). Periodic testing of rats that came in after the full quarantine will aid in determining if this has happened.

Prevention

• Test new rats before admitting them into the colony (Could be done, but as an antibody positive rat isn’t necessarily infectious, it may make more sense to PCR test to make sure no active shedding is present in the new rats).
• Receive rats from RV-free colonies (There’s no data on actual prevalence of RV in the pet population, or at least not in the UK, but watch this space, as those in the UK may be about to conduct a seroprevalence study!).
• Wild rats can carry RV, so it is recommended to keep your domesticated rats in an area where wild rats have no access.
• Quarantining new rats would help you to avoid RV only if it is extended past the typical 2–3-week quarantine (60–90 days for adults and up to six months for rats exposed in utero or shortly after birth) and may not provide a practical solution.

Consultation and Technical Assistance

• Dr. Craig L. Franklin, DVM, Ph.D., ACLAM, University of Missouri Research Animal Diagnostic and Investigative Laboratory (IDEXX RADIL). Phone and email consultation.
• Dr. Robert Jacoby DVM, Ph.D., Dipl. ACVP, Professor and Chairman, Section of Comparative Medicine Yale University School of Medicine. Phone consultation.
• Dr. Abigail L. Smith, MPH, PhD, Director, Laboratory Animal Science, Jackson Laboratory, Bar Harbor, ME, President American Committee on Laboratory Animal Diseases (ACLAD). Email consultation and article proofing.
• Charles River Laboratories- Technical Assistance Department. Phone consultation.
• Adele Wharton, BVSc, MRCVS, CertGP,Veterinary Surgeon, Nottinghamshire, United Kingdom. Co-authored recent updates and updated article proofing

References

1. Ball-Goodrich, L., Paturzo, F., Johnson, E., Steger, K., & Jacoby, R. (2002). Immune responses to the major capsid protein during parvovirus infection of rats. J Virol, 76(19), 10044-9. Retrieved December 23, 2008, from the Medline database.
2. Ball-Goodrich, L., Leland, S., Johnson, E., Paturzo, F., & Jacoby, R. (1998). Rat parvovirus type 1: the prototype for a new rodent parvovirus serogroup. J Virol, 72(4), 3289-99. Retrieved December 23, 2008, from the Medline database.
3. Barthold, S., & Percy, D. (2001). Pathology of Laboratory Rodents and Rabbits, Second Edition. Ames, Iowa: Iowa State Press.
4. Jacoby, R., Johnson, E., Paturzo, F., Gaertner, D., Brandsma, J., & Smith, A. (1991). Persistent rat parvovirus infection in individually housed rats. Arch Virol, 117(3-4), 193-205. Retrieved December 23, 2008, from the Medline database.
5. Jacoby, R., Ball-Goodrich, L., Besselsen, D., Mckisic, M., Riley, L., & Smith, A. (1996). Rodent parvovirus infections. Lab Anim Sci, 46(4), 370-80. Retrieved December 23, 2008, from the Medline database.
6. Jacoby, R., Ball-Goodrich, L., Paturzo, F., & Johnson, E. (2001). Prevalence of rat virus infection in progeny of acutely or persistently infected pregnant rats. Comp Med, 51(1), 38-42. Retrieved December 23, 2008, from the Medline database.